Olefinic polymers such as polyethylene, polypropylene, polyvinyl chloride, polystyrene and polyesters are subject to photo-oxidation when exposed to sunlight over extended periods of time. This photo-oxidation initiates deterioration of the polymer by breaking the polymer chain and by causing formation of carbonyl groups in the molecule. As the oxidation continues, the polyolefin cracks or crazes and loses tensile strength to the point of mechanical failure.
Polyacrylamides, polyamides, polyacrylates and polycarbonates are also affected by ultraviolet radiation exposure such that they tend to discolor and darken; further, certain dyed fibers of these polymers have a tendency to fade under prolonged exposure to daylight. Accordingly, avoidance of these problems by economical incorporation of a suitable stabilizer is highly desirable.
Short wavelengths of ultraviolet radiation are capable of causing photochemical damage to coatings and the surface layers of objects that comprise organic compounds. Exterior surface coatings, such as exterior paints and varnishes, are normally exposed to sun light several hours each day and, therefore, are particularly subject to damage by ultraviolet radiation rays. The shortest wavelength, and most damaging, radiation is in the ultraviolet (UV) range. Wavelengths below 300 nm (nanometers) are the most destructive of the ultraviolet radiation. Most of the ultraviolet rays in this very short wavelength range are absorbed by the atmosphere but sunlight contains ultraviolet radiation in the range of 300 nm to 400 nm is also very destructive of organic materials such as coatings and the surface layer of objects formed of organic compounds. Ultraviolet radiation in the 300 nm to 400 nm range passes through the atmosphere and causes both structural damage and the fading or darkening of colors. Colors often fade and clear coatings and materials tend, generally, to become yellow or darker colored. Even above the ultraviolet range, visible light in the 400 nm to 500 nm wavelength range causes some fading or darkening of colors; however, the very short wavelengths are much more damaging. For example, ultraviolet radiation in the 300 nm wave-length range is approximately 200 times more damaging than visible light in the 500 nm wavelength.
UV absorbers or quenchers are added to most coatings and other organic materials, organic polymers typically, that are expected to be exposed to ultraviolet radiation. The photochemical responses of true absorbers differs from that of true quenchers, but both may be characterized as "absorbers" in the sense used herein, i.e. reaction with ultraviolet radiation to prevent the ultraviolet radiation from reacting with the principle polymer to which the absorber is added. The chemistry of ultraviolet absorbers or quenchers is fairly well known and several classes of ultraviolet absorbing compound are known.
Ultraviolet stabilizers are organic compounds used to protect polymers and coatings from photo degradation induced by sunlight and artificial sources of ultraviolet radiation. They are classified into ultraviolet radiation absorbers or quenchers and ultraviolet radiation-stable antioxidants. The former include derivatives of 2-hydroxybenzophenone and 2-hydroxybenzotriazote, and the latter derivatives of hindered amines. Usefulness depends on optical properties, antioxidant effectiveness, long-term stability, solubility in the substrate, volatility, and compatibility with other additives. The following are examples of the numerous ultraviolet absorbers or quenchers known in the art: 1,2-[Bis(3,3,4,5,5-pentamethyl-2oxo-1-piperazinyl)]ethane; 2-(2'-Hydroxy-3',5'-di-t-butyl)-5-chlorobenzotriazole; 2-(2'-Hydroxy-5-methylphenyl)benzotriazole; 2-(2'Hydroxy-5'-t-octylphenyl)benzotriazole; 2-(3'-t-Butyl-2'-hydroxy-5'-methylphenyl)-5-chlorobenzotriazole; 2-[2'-Hydroxy-3',5'-(di-t-amyl)phenyl]benzotriazole; 2-[2'-Hydroxy-3',5'-(di-t-butyl)phenyl]benzotriazole; 2-[2'Hydroxy-3',5'-di-(.alpha.,.alpha.-dimethylbenzyl)phenyl]benzotriazole ; 2-Ethyl-2'ethoxy-5'-t-butyloxalanilide; 2-Ethyl-2'-ethoxyoxalanilide; 2-Ethylhexyl 2-cyano-3,3-diphenylacrylate; 2-Hydroxy-4-dodecyloxybenzophenone; 2-Hydroxy-4-isooctoxybenzophenone; 2-Hydroxy-4-methoxy-5-sulfobenzophenone, 2Hydroxy-4-methoxybenzophenone; 2-Hydroxy-4-n-octoxybenzophenone, 2,2-Dihydroxy-4,4'-dimethoxybenzophenone; 2,2'-Dihydroxy-4-methoxybenzophe none; 2,2'Thiobis(4-t-octylphenolato-n-butylamine nickel; 2,2',4,4'-Tetrahydroxybenzophenone; 2,4-Di-t-butylphenyl-3,5-di-t-butyl-4-hydroxybenzoate; 2,4-Dihydroxybenzophenone; and 3,5-Di-t-butyl-4-hydroxybenzoic acid.
Specifications for several classes of ultraviolet radiation absorbers suitable for use in the present invention are available in the literature and may be obtained from manufactures; e.g., SANTASE.RTM. (Neville-Synthese Organics, Inc.), CYSORB UV.RTM. (American Cyanamid Co.), UVINUL.RTM. (BASF Wyandotte Corp.), TINUVIN.RTM. (Ciba-Geigy Corp.), SANDUVOR.RTM. (Sandoz Color and Chemicals), SALOL.RTM. (Dow Chemical Co.) and EASTMAN RMB.RTM. (Eastman Chemical Co.).
Dodecyl-hydroxyphenyl benzotriazole isomeric mixtures, preferably the 2-(5'-dodecyl-2'-hydroxyphenyl) benzotriazole isomeric mixtures are known to stabilize polymer systems such as are used in spar varnishes. Such stabilizers may additionally contain antioxidant, for example, a sterically hindered phenol such as 2,6-ditertiarybutyl-4-methylphenol (Ionol); dilaurylthiopropionate; distearylthiodipropionate; etc., or any other useful anti-oxidant. Alternatively, the stabilizer mixture can be incorporated into a final polymeric formulation including the anti-oxidant and other additives. When employed, between about 0. 1 weight percent and about 5 weight percent, more usually 0.25 to 1.5 weight percent, of anti-oxidant based on oxidizable species is suitable. Such stabilizers provide protection from ultraviolet degradation in lacquers, paints and varnishes. Specifically "chalking" of finishes subjected to constant exposure to the elements can be greatly reduced. Such stabilizers can be incorporated in concentrations as high as 10% to 15% (by weight) to provide an exceptionally resistant finish, though it is rare that more than about 5% would be beneficial. Solvents suitable for stabilizers include low and high boiling water immiscible organic solvents and low boiling water soluble organic solvents and dispersants and intermixtures of solvents. Examples of these include lower alcohols of 1 to 4 carbon atoms and containing one or more hydroxy groups, for example, ethanol, methanol, propanol, isopropanol, glycerol, glycol, etc; lacquer vehicles; oils, for example, white petrolatum, paraffin oil, linseed oil, castor oil, oil of lemon, oil of caraway, oil of spearmint, oil of rose, mineral or vegetable oils, such as the polyoxyethylated vegetable oils; olive oil, glycerin, vaseline, cocoa butter, lanolin, light petroleum oils or lubricating oils; tincture of benzoin; aromatic solvents, for example, lower alkyl p-aminobenzoate, benzene, toluene or xylene; higher alcohols; phenol, dimethylphenol, resorcinol; ketones, such as methyl ethyl ketone, ethyl ketone, acetone; alkyl pyrrolidones; cycloaliphatic hydrocarbons, for example, cyclohexane; pyrogallic acid; fatty esters; water based emulsions; alkoxy alkylacetates; ethyl-and butyl-cellusolves; ethylene glycol; terpentine; bisphenols; hydroxy biphenols; triphenyl phosphate and other organic phosphates; sterically hindered phenols, for example, Ionol; phthalates for example, benzylphthalate and dibutylphthalate; ethers, for example Cellosolve, Solox and many others.
For optimum protection, it is desirable to absorb radiation up to 400 nm. An ideal ultraviolet absorber or quencher should absorb the radiation between 290 and 400 nm while transmitting all visible light. Absorption directly above 400 nm causes yellowing of the substrate. No ultraviolet absorber or quencher has this square-wave type of absorption, although substituted 2-(2'-hydroxyphenyl)benzotriazoles approach this ideal most closely. These compounds absorb very strongly throughout most of the ultraviolet region and show a rapid decrease in absorbance approaching 400 nm. Compounds with little or no absorbance beyond 400 nm are preferred for most polymer applications.
Those skilled in the art have always believed that, for optimum screening activity, the absorber or quencher should be molecularly dispersed in the substrate. Incomplete solubility is known, generally, to result in a lower absorbance than the theoretical value calculated from the Beer-Lambert law and that poor solubility also results in exudation of the additive to the polymer surface.
Ultraviolet absorbers or quenchers with extremely low volatility are required for polymers that are extruded at high temperatures. Polycarbonate and poly(ethylene terephthalate) are processed at temperatures of ca 300.degree. C. A considerable amount of the additive is lost when the hot polymers are exposed to the atmosphere unless the additive has a very low vapor pressure. Low volatility is also required for applications such as automotive paints where the stabilizer must suffer only minimal loss during oven drying and outdoor exposure. Volatility can be minimized in a number of ways, such as the introduction of appropriate substituents as in the case of 2-[2'-hydroxy-3',5'-di(.alpha.,.alpha.dimethylbenzyl)]phenylbenzotriazole, or chromophores that absorb ultraviolet can be grafted onto the backbones of preformed polymers, built into the polymer chain by condensation, or addition polymerization.
Acrylic polymers are among the most commonly used coating material constituents and are also among the least susceptible to degradation by ultraviolet radiation. Transparent acrylic polymers that are opaque to ultraviolet radiation may be prepared by incorporating ultraviolet absorbers or quenchers in photo-stable acrylics such as polymethylmethacrylate (PMMA). However, the ultraviolet absorber or quencher is lost from the acrylic polymer by evaporation, transfer to other materials where physical contact occurs, such as the use of cleaning cloths, or by being washed out by rain, spray or other contact with liquids. Discoloration, embrittlement, chalking, cracking and loss of mechanical properties result from ultraviolet degradation as the ultraviolet absorber or quencher is removed by evaporation, leaching or partitioning into a liquid phase.
Polyolefins, polystyrenes, polyvinyl chlorides, unsaturated polyesters, polyurethanes, polycarbonates, and polyamides are all subject to discoloration and/or mechanical property degradation by ultraviolet radiation. Acrylics are resistant to ultraviolet degradation but the addition of a ultraviolet absorber increases the long-term weatherability of even acrylics.
Acrylate-, epoxy- and urethane-based varnishes are more resistant to weathering than most coatings, but they degrade over a period of time, sometimes only a few months, and some times a few years, when exposed to the cycling of temperature, moisture and ultraviolet radiation to which exterior surfaces are exposed. Urethane- and epoxy-based varnishes, especially, form sparkling clear, water resistant coatings but are subject to fairly rapid ultraviolet degradation, even though they contain from about 0.1 to 1 percent, by weight, ultraviolet absorber. Some special varnishes are formulated with as high as 3 to 5 weight percent ultraviolet absorber and sold at a premium price. Even with high concentrations of ultraviolet absorber, these varnishes degrade in the presence of ultraviolet radiation over several months.
To the best of the inventor's knowledge, no fully adequate solution to the problem of degradation in marine and other water resistant varnishes has been found. It is to the solution of this problem in particular, and to the improvement of polymer coatings and castings generally, that the present invention is directed.